As much importance has been attached to saving in resources and countermeasures to environment in recent years, requirements to reduce fuel consumption of automobiles have been increasingly severe. Automobile tires have also been required to lower their rolling resistance so as to contribute to reduction in the fuel consumption. In order to lower the rolling resistance of tires, it is generally effective to use a rubber material capable of providing vulcanized rubber having low heat build-up as a rubber material for the tires.
In general, rubber compositions in which carbon black is compounded into a diene rubber such as natural rubber (NR), polybutadiene (BR), polyisoprene (IR) or styrene-butadiene copolymer rubber (SBR) are widely used as rubber materials for tires. However, the rubber compositions obtained by compounding carbon black into the diene rubber have insufficient heat build-up resistance.
It has heretofore been proposed to use, as a rubber material for tires, a rubber composition with silica compounded in place of carbon black as a reinforcing agent into a diene rubber so as to reduce heat build-up. However, the silica-compounded rubber composition has involved a problem that its abrasion resistance and tensile strength are insufficient compared with the carbon black-compounded rubber composition. A cause for this problem is considered to be attributed to the fact that the affinity of the silica for the diene rubber is lower than that of the carbon black, so that a reinforcing effect cannot be sufficiently developed.
In order to enhance the affinity of the silica for the diene rubber, it has heretofore been investigated to use a diene rubber with a substituent group having high affinity for silica introduced therein. For example, a diene rubber with a tertiary amino group introduced therein has been proposed for a diene rubber according to an emulsion polymerization process (Japanese Patent Application Laid-Open No. 101344/1989). Besides, a diene rubber with a substituent group such as an alkylsilyl group (Japanese Patent Application Laid-Open No. 188501/1989), a halogenated silyl group (Japanese Patent Application Laid-Open No. 230286/1993) or a substituted amino group (Japanese Patent Application Laid-Open No. 22940/1989) introduced therein has been proposed for a diene rubber according to an anionic polymerization process.
However, many of the diene rubbers with these substituent groups introduced therein have demerits that their processability is poor upon blending with silica, and their properties such as heat build-up resistance, tensile strength and abrasion resistance are not sufficiently improved.